The present invention relates to forming protective coatings on turbine engine components that are exposed to high temperature, oxidation and corrosive environments. More particularly, the invention is directed to forming a protective coating useful as an environmental coating or a bond coat for a thermal barrier coating on turbine engine components, by depositing at least two platinum group metals on the substrate using an electroplating process.
In an aircraft gas turbine engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on a shaft. The flow of gas turns the turbine, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the exhaust gases, the more efficient are the operation of the jet engine. There is thus an incentive to raise the exhaust gas temperature. However, the materials used to fabricate the turbine vanes and blades normally limit the maximum temperature of the exhaust gases. In current engines, the turbine vanes and blades are made of nickel-based superalloys that can operate at temperatures of up to about 1150° C.
Many approaches have been used to increase the operating temperature limit of turbine blades and vanes. The composition and processing of the materials used to form the blades and vanes have been improved. Physical cooling techniques have also been used, such as forcing air through internal cooling channels of the components during engine operation.
In another approach, an environmental coating or a thermal barrier coating system is applied to turbine blades or vanes. The thermal barrier coating system includes a ceramic thermal barrier coating that insulates the component from the hot exhaust gas, permitting the exhaust gas to be hotter than would otherwise be possible with the particular material and fabrication process of the component. Ceramic thermal barrier coatings usually do not adhere well directly to the superalloy substrates. Therefore, an additional metallic layer called a bond coat is placed between the substrate and the thermal barrier coating. The bond coat is usually made of an aluminum-containing overlay alloy, such as a NiCrAlY or a NiCoCrAlY, or of a diffusion nickel aluminide or platinum aluminide material.
While superalloy components coated with a thermal barrier coating system provide substantially improved performance over uncoated components, there remains room for further improvement in resistance to oxidation and hot corrosion damage. In addition, the alloying elements of conventional environmental coatings and bond coats can interdiffuse with the substrate alloy and produce brittle intermetallic phases. Thus, there is an ongoing need for protective coatings for turbine engine components that have improved environmental resistance and long-term stability when used as an environmental coating or as a bond coat for a thermal barrier coating system.